Nervous system disorders affect millions of people, causing death and a degradation of life. Nervous system disorders include disorders of the central nervous system, peripheral nervous system, and mental health and psychiatric disorders. Such disorders include, for example without limitation, epilepsy, Parkinson's disease, essential tremor, dystonia, and multiple sclerosis (MS). Additionally, nervous system disorders include mental health disorders and psychiatric disorders which also affect millions of individuals and include, but are not limited to, anxiety (such as general anxiety disorder, panic disorder, phobias, post traumatic stress disorder (PTSD), and obsessive compulsive disorder (OCD)), mood disorders (such as major depression, bipolar depression, and dysthymic disorder), sleep disorders (narcolepsy), obesity, and anorexia. As an example, epilepsy is the most prevalent serious neurological disease across all ages. Epilepsy is a group of neurological conditions in which a person has or is predisposed to recurrent seizures. A seizure is a clinical manifestation resulting from excessive, hypersynchronous, abnormal electrical or neuronal activity in the brain. (A neurological event is an activity that is indicative of a nervous system disorder. A seizure is a type of a neurological event.) This electrical excitability of the brain may be likened to an intermittent electrical overload that manifests with sudden, recurrent, and transient changes of mental function, sensations, perceptions, and/or involuntary body movement. Because the seizures are unpredictable, epilepsy affects a person's employability, psychosocial life, and ability to operate vehicles or power equipment. It is a disorder that occurs in all age groups, socioeconomic classes, cultures, and countries. In developed countries, the age-adjusted incidence of recurrent unprovoked seizures ranges from 24/100,000 to 53/100,000 person-years and may be even higher in developing countries. In developed countries, age specific incidence is highest during the first few months of life and again after age 70. The age-adjusted prevalence of epilepsy is 5 to 8 per 1,000 (0.5% to 0.8%) in countries where statistics are available. In the United States alone, epilepsy and seizures affect 2.3 million Americans, with approximately 181,000 new cases occurring each year. It is estimated that 10% of Americans will experience a seizure in their lifetimes, and 3% will develop epilepsy by age 75.
There are various approaches in treating nervous system disorders. Treatment therapies can include any number of possible modalities alone or in combination including, for example, electrical stimulation, magnetic stimulation, drug infusion, and/or brain temperature control. Each of these treatment modalities can be operated using closed-loop feedback control. Such closed-loop feedback control techniques receive from a monitoring element a neurological signal that carries information about a symptom or a condition or a nervous system disorder. Such a neurological signal can include, for example, electrical signals (such as EEG, ECoG, and/or EKG), chemical signals, other biological signals (such as change in quantity of neurotransmitters), temperature signals, pressure signals (such as blood pressure, intracranial pressure or cardiac pressure), respiration signals, heart rate signals, pH-level signals, and peripheral nerve signals (cuff electrodes on a peripheral nerve). Monitoring elements can include, for example, recording electrodes or various types of sensors.
For example, U.S. Pat. No. 5,995,868 discloses a system for the prediction, rapid detection, warning, prevention, or control of changes in activity states in the brain of a patient. Use of such a closed-loop feed back system for treatment of a nervous system disorder may provide significant advantages in that treatment can be delivered before the onset of the symptoms of the nervous system disorder.
Monitoring of a neurological signal for the detection of a nervous system disorder, however, has its difficulties. For example, the monitoring system can become degraded for various reasons including, for example, a broken connection, a faulty contact, external disturbance, degradation of the line, excessive electrical noise from devices (caused by faulty grounding, too many devices, etc.), movement of the wires causing inductance, patient moving around and causing displacement of components relative to each other, other physiologic generators such as blinking and eye movements, EKG, glossokinetic potential (swallowing or tongue movements), etc.
To account for such degradation, for example, U.S. Pat. No. 5,995,868 discloses the possibility of utilizing online signal quality control methods to detect various form of signal degradation and warn the user or others. As another example, J. Gotman discloses a system that performs pre-filtering and artifact rejection of EEG signals for purposes of detecting a seizure. J. Gotman, “Automatic Recognition of Epileptic Seizures in the EEG,” Electroencephalography and clinical Neurophysiology, Vol. 54: pp. 530–540 (1982). Gotman discloses the use of a fixed control using a small time window. For example, if the received data is beyond a threshold amplitude for sample points over all channels, the system cancels all detections in all channels for the current and previous epoch.
Since Gotman utilizes a fixed time window wherein a single data point is considered, an instantaneous determination is made whether or not to ignore a given signal over an epoch. It may be desirable, however to consider data points over a given period of time. Instantaneous and piecemeal determinations may not provide a sufficiently accurate and flexible technique for detecting and removing from consideration channels have poor signal quality.
Thus, it would be an advancement in the art to provide a method or apparatus that can monitor received signal and determine based on signal quality over a period of time whether signal degradation has occurred.